1. Field of the Invention
The present invention relates to a novel photomask material and a method for producing same.
2. Description of the Prior Art
Photomasks having high resolving power are necessary in the production of, for example, the so-called integrated circuits (IC). As the photomask material for producing photomasks, a photographic material comprising a transparent support having coated thereon a silver halide photographic emulsion, in which material silver halide images are formed to prepare a photomask (hereinafter referred to as an emulsion mask) has been used. A large amount of this type of material is still being widely used.
However, an emulsion mask has many disadvantages in that the image layer is mechanically weak, the silver image is opaque to visible light as well, and edge acuity is poor due to the grain property of silver.
Therefore, an excellent photomask without these disadvantages has been desired in this field. The so-called hard mask, represented by a chromium photomask, has been developed to satisfy these requirements. At present, a chromium oxide mask, an iron oxide mask, a silicon mask, etc., are also known as well as the above-described chromium mask. These masks, except for the chromium mask, provide semi-transparent mask images (with these masks usually being called "see-through masks") and these hard masks are extremely strong and durable. However, in comparison with the above-described emulsion mask, these masks possess the following defects. That is, since the hard mask material itself is not light-sensitive, a photoresist must be coated thereon to utilize the light sensitivity of the photoresist, in order to prepare a hard mask using a hard mask material. However, the sensitivity of the photoresist is usually on the order of about ASA 10.sup.-.sup.5, which is far smaller than the sensitivity of an emulsion mask having a sensitivity on the order of ASA 10.sup.-.sup.2. This poor sensitivity requires a prolonged exposure time upon exposing this photomask material using a photo-repeater. Furthermore, a hard mask has the defect that the surface reflection of the mask is so great that light reflected on the surface upon imagewise exposure diffuses within the photoresist layer to reduce the resolving power. Another defect with a hard mask is that, since the mask imageforming layer projects on the support, the mask layer is often chipped when the mask is pressed against a semi-conductor wafer, with the mask image being worn away. This phenomenon naturally leads to a reduction in the life of the hard mask.
Therefore, the development of a photomask which possesses both the advantages of an emulsion mask (i.e., a high sensitivity and low surface reflection) and the advantages of a hard mask (i.e., great durability, see-through ability and good edge acuity) has been strongly desired.
Investigations have now been made to apply the art relating to photographic materials, such as "MetalPhoto", "Alphoto" or like trade named materials, to photomask use. This photographic material is prepared by subjecting the surface of an aluminum plate to anodic oxidation to form an aluminum oxide film of a thickness of several microns to several tens of microns and filling the fine pores of about several hundred angstroms in size, formed in the oxide film, with silver halide (hereinafter referred to for brevity as "aluminum photo material").
With this photographic material, silver images can be formed by subjecting the material to development and fixing processing after imagewise exposure, in a similar manner to ordinary silver halide photographic materials.
As is well known, a photomask must have the property that non-image areas are transparent to the light to which the photoresist is sensitive. However, since the above-described aluminum photo material has an opaque support of an aluminum plate, the material cannot be utilized as a photomask. Therefore, attempts were made to use a material comprising a transparent support having thereon an aluminum thin film layer, in place of the aluminum plate and to subject the thin film layer to anodic oxidation to thereby render the thin film layer transparent. However, according to these investigations, the above attempt cannot actually be practiced with ease due to the following two important problems. Firstly, in subjecting a thick aluminum plate to anodic oxidation, no problems occur because the aluminum base plate functions as an electrode (anode), whereas in subjecting a thin aluminum film (e.g., several microns in thickness) provided on an insulating support to anodic oxidation, the phenomenon occurs that, when anodic oxidation is initiated using the thin aluminum layer as an anode, the aluminum film is completely converted to an oxide film at some portions while at other portions aluminum remains in the form of islands. Although an aluminum oxide film is first formed all over the surface of the aluminum thin layer and then the oxidation gradually proceeds to the interior portion, the oxidation rate is not completely uniform over the entire aluminum thin film layer but differs due to the nature of the portion. If islands of aluminum remain, these areas are insulated and oxidation does not proceed further. As a result, this portion remains opaque. Secondly, the intimate adhesiveness between the oxide film and the support is a problem. Even when aluminum is intimately adhered to the support, the aluminum layer sometimes changes in volume upon conversion of aluminum to aluminum oxide. Thus the intimate adhesiveness between the aluminum oxide layer and the support is degraded.